The present invention relates to a new structure of a light emission device used for, e.g., a display device, and specifically to a molecular light emission device at least comprising an electrode for injecting electrons, an electrode for injection holes, and an organic molecule emitting light by electric current injection, wherein an organic molecular layer formed between the electrodes consists essentially of a single (i.e.,monolayer) molecule. In a variation of the present invention, the function of the interior of the organic molecule emitting light by the electric current injection may be divided into the following: functions of an electron transport layer, a light emission layer, a hole transport layer and the like, but the present invention relates to a molecular emission device characterized in that the molecule itself consists essentially of a single (i.e., monolayer) molecule.
In conventional stack-type molecular light emission device comprising organic molecules (referred to as stack-type organic light emission devices, or stack devices, hereinafter), conjugate molecules composed mainly of aromatic groups are deposited, as an electron transport layer, a light emission layer and a hole transport layer, in this order from the side of an electron injection electrode and to each thickness of several tens of nanometers, and finally a hole injection electrode is formed. (The outline of the structure thereof is in detail described in, for example, Bulletin of the Japanese Society of Printing Science and Technology, Vol. 34, No. 4 (1997), pp. 218-226). Electrons and holes are respectively injected from both of the electrode, i.e., the electron injection electrode and the hole injection electrode, and then recombined in the light emission layer to emit light. The details of organic light emission devices having conventional structures are described in IEEE Trans. Electron Devices, Vol. 44 No. 8 (1997), and pp. 1188-1203 and the like.
If it is possible to put into practical use a display device having excellent characteristics such as thinness, light emitting ability, flatness and flexibility as the stack-type molecular light emission devices exhibit, a new industrial field can be realized as a high-performance display device instead of a liquid crystal display device. Such a device has been much expected.
However, such stack-type organic light emission devices having a conventional structure have various problems from the viewpoint of practicability even at the present time, when 30 years have passed since the discovery of the phenomenon concerned with the devices. Thus, the devices are far from practicability. Current problems of the stack-type organic light emission devices will be described in the following items.
(1) The contact resistance between the injection electrodes and the molecules is so large that the efficiency of injecting electrons or holes is low. In other words, ohmic contact cannot be realized.
(2) Since the mechanism of electric conduction between molecules is based on tunnel conduction or hopping conduction, conduction efficiency is very low. Thus, injected carries are recombined on their way, so that they do not necessarily contribute to light emission.
(3) The probability is low that the carries reaching the center of light emission excite molecules which contribute to the light emission. Thus, light emission efficiency does not become high.
(4) Light emitted from excited molecules is absorbed or reflected on its way, so that the probability that the light is not emitted outwards from the device is high. Thus, overall light emission efficiency is low.
According to Tsutsui(Oyo-butsuri, Vol. 66, No. 2, 1997, pp. 109-116), theoretical light emission efficiency of stack devices is 5%. Since practical efficiency is considered to be far lower than the theoretical efficiency, the practical efficiency is remarkably lower than that of current light emission devices, such as 25% of a fluorescent light lamp. Thus, stack devices of a present situation are far from practicability.
In order to solve such problems of conventional light emission devices comprising an organic molecule, the present invention has been made and provides means for solving the problems in the above-mentioned four items at once. That is, the present invention has the following characteristics.
(1) Contact resistance between an injection electrode and a molecule layer is made small so that the injection efficiency of electrons and holes into the molecules becomes very high.
(2) The conduction mechanism of the molecule layer is made ohmic to raise conductive efficiency.
(3) Almost 100% of carries that reach the center of light emission would contribute to light emission.
(4) Light emitted from excited molecules goes outwards at a high probability.
For this, the present invention provides a device having a structure wherein a single light emission molecule is chemically bonded directly to an electron injection electrode and a hole injection electrode, which is quiet different from the stack-type device using layers of molecules, such as an electron transport layer, a light emission layer and a hole transport layer, each made up of different properties. This single light emission molecule may have electron transport, electron-hole recombination (ie. light emission), hole transport and the like functions. Namely, in order to realize the high efficiency organic light emission device, it is sufficient that a single molecule having at least light emission function is connected to each surface of two electrodes through chemical bonds.
Such a structure is referred to as a single organic molecular light emission device or a single device, hereinafter.
The following will describe the reasons why the single molecule is connected to the two electrodes through chemical bonds so that a single organic molecular light emission device having a high light emission efficiency can be realized.
(1) The reason why the contact resistance between a carrier injection electrode and a molecule is high in conventional stack-type devices is that any chemical bond between the electrode and the molecule cannot be produced. Since in stack-type devices a molecule layer is in general formed on its electrode in air or low vacuum, an oxide layer on the surface of the electrode, contamination from the surroundings, and the like are produced between the electrode and the molecule layer. This results in a large resistance. Furthermore, the molecule structure used in conventional stack-type devices has a substituent which cannot be chemically bonded to the electrode. Accordingly, if a chemical bond can be produced between the electrode and the light emission molecule, contact resistance can be made sufficiently low. In the single device according to the present invention, a chemical bond can be produced between the electrode and the light emission molecule. Thus, its contact resistance can be made sufficiently low.
(2) The reason why conduction efficiency in a molecule layer in stack-type devices is low is that respective molecules are electrically isolated and that they are not chemically bonded to each other. Therefore, if both electrodes are connected through a single molecule, a high conductivity in the light emission layer sufficient for practicability can be ensured because of a satisfactorily high conductive efficiency in the single molecule.
(3) The reason why excited molecules are inactivated through a non-light emission process is considered to be based mainly on emission of phonons. It is well known to the public that there is a great probability that excited molecules are inactivated through a phonon transition process by interaction between molecules in stack-type devices. On the other hand, it is predicted that in the single device the process in which the excited state of molecules is deactivated to the ground state thereof arises at a probability determinable by the a probability process. Thus, this probability can be a physical and chemical maximum value. Therefore, the single device structure is also preferred from the standpoint of this aspect.
(4) In stack-type devices, light emitted from excited molecules is again absorbed into adjacent molecule, so that light emission efficiency is reduced. On the other hand, it is considered that in the single device its emitted wavelength is longer than its absorbable wavelength. Thus, it is expected that a decrease in light emission efficiency by such absorption is reduced.
As describe above, the single device structure can overcome the basic problem of low efficiency in conventional stack-type devices. Thus, the single device structure is a technical means useful for putting an organic light emission device to practical use. Moreover, because of high efficiency the single device structure can make electric field strength sufficiently low so as so that this device structure which is also practicable from the standpoint of reliability.